1. Field of the Invention
The present invention relates to a method and an apparatus for deciding a heated state of a billet made of metal, for detecting the heated state of the metal billet during the transport of the heated metal billet to a forming machine.
2. Description of the Related Art
A method is generally known, in which a metal preform (hereinafter referred to as "metal billet") is heated to be in a semisolidified state by using a heating apparatus, and then the metal billet is subjected to forming operation by using a forming apparatus while maintaining the metal billet in a semisolidified state to form a predetermined formed product of metal.
The heating apparatus to be used for such a method comprises a plurality of induction heaters corresponding to the billet, the induction heaters being disposed over a placing apparatus provided with a series of insulated pedestals arranged around a rotatable table, as disclosed, for example, in Japanese Patent Publication No. 2-7748. The induction heater is constructed such that when the billet is guided to arrive at the position of the induction heater in accordance with the rotation of the table, the billet, which stands without being supported at its side surface, is allowed to have an enthalpy which is successively increased to a level at which the billet is partially in a semisolid state.
The billet, which is taken out of the heating apparatus, is subjected to predetermined forming operation by using the forming apparatus. Therefore, in order to obtain a high quality formed product by using the forming apparatus, it is necessary to maintain the billet to be in a predetermined heated state, i.e., to have a predetermined solidus rate. Accordingly, a variety of operations have been hitherto carried out in order to detect whether or not the billet after completion of the heating is maintained to have a predetermined solidus rate.
Those hitherto adopted include, for example, a method in which a billet after completion of the heating is arbitrarily extracted or sampled, and the billet is cut with a knife or the like to judge whether the heated state is good or bad (hereinafter referred to as "first method"), a method in which a thermocouple is previously inserted into a billet before mass-production, and the heating condition is determined on the basis of a result of temperature measurement, obtained by using the thermocouple (hereinafter referred to as "second method"), and a method in which the heated state of a billet is judged from inclination and deformation of the billet by means of visual observation performed by an operator (hereinafter referred to as "third method").
However, in the case of the first method described above, it is impossible to perform on-line feedback control for external factors such as atmospheric temperature and water temperature, because the billet is subjected to sampling inspection. Therefore, it is difficult to stably maintain the product quality. In the case of the second method described above, it is impossible to quantitatively manage the condition during mass-production, because the condition is previously determined before the mass-production. In the case of the third method described above, the production line is sometimes stopped in order to confirm the inclination and deformation of the billet. Therefore, a problem is pointed out that the productivity is extremely deteriorated.